Post-Contrast Myocardial T₁ and ECV Disagree in a Longitudinal Canine Study

Abstract

Both post-contrast myocardial T₁ and extracellular volume (ECV) measurements have been associated with interstitial fibrosis. The cardiovascular magnetic resonance (CMR) field is migrating towards ECV, because it is largely insensitive to confounders that affect post-contrast myocardial T₁. Despite the theoretical advantages of myocardial ECV over post-contrast myocardial T₁, systematic experimental studies comparing the two measurements are largely lacking. We sought to measure the temporal changes in post-contrast myocardial T₁ and ECV in an established canine model with chronic atrial fibrillation.

Seventeen mongrel dogs, implanted with a pacemaker to induce chronic atrial fibrillation via rapid atrial pacing, were scanned multiple times for a total of 46 CMR scans at 3T. These dogs with different disease durations (0–22 months) were part of a separate longitudinal study aimed at studying the relationship between AF and patho-physiology. In each animal, we measured native and post-contrast T₁s and hematocrit. Temporal changes in post-contrast myocardial T₁ and ECV, as well as other CMR parameters, were modeled with linear mixed effect models to account for repeated measurements over disease duration.

In 17 animals, post-contrast myocardial T₁ decreased significantly from 872 to 698 ms (p< 0.001), which corresponds to a 24.9% relative reduction. In contrast, ECV increased from 21.0 to 22.0% (p=0.38), which corresponds to only a 4.5% relative increase. To partially investigate this discrepancy, we quantified collagen volume fraction (CVF) in post-mortem heart tissues of 6 canines sacrificed at different disease duration (0–22 months). CVF quantified by histology increased from 0.9 to 1.9% (p=0.56), which agrees more with ECV than post-contrast myocardial T₁. This study shows that post-contrast myocardial T₁ and ECV may disagree in a longitudinal canine study. A more comprehensive study, including histologic, cardiac, and renal functional analyses, is warranted to test rigorously which CMR parameter (ECV or post-contrast myocardial T₁) agrees more with CVF.

Introduction

Diffuse myocardial fibrosis is a well-established marker of adverse structural remodeling in a variety of heart diseases, including: atrial fibrillation (AF) (1), heart failure (2), hypertrophic cardiomyopathy (3,4), aortic stenosis (3,4), cardiac amyloidosis (3,4), myocardial infarction (4), diabetes (5), and congenital heart disease (6). Cardiovascular magnetic resonance (CMR) methods such as post-contrast cardiac T₁ mapping (7–9) and extracellular volume (ECV) mapping (4,9–12), derived from native and post-contrast myocardial and blood T₁ measurements, are the only validated non-invasive tests for interstitial fibrosis. The CMR field is recognizing that post-contrast myocardial T₁ measurement is sensitive to a variety of confounders, including: renal function, hematocrit, magnetic field strength, contrast agent type and dosage, and specific delayed imaging time after administration of contrast agent. To account for these confounders, many investigators in the CMR field are migrating toward myocardial ECV as a marker of diffuse myocardial fibrosis (4,6,10–13). Despite the theoretical advantages of ECV over post-contrast myocardial T₁ (14), systematic experimental studies comparing the two measurements are largely lacking, particularly in a longitudinal setting.

Animal models provide a unique opportunity to perform a longitudinal study to monitor the temporal changes in left ventricular (LV) structural remodeling. We have established a canine model with chronic AF to study the relationship between AF and cardiac pathophysiology (15). We sought to leverage this longitudinal canine study and compare the changes in post-contrast myocardial T₁ and ECV measurements over disease duration.

Materials and Methods

Animal Preparation for CMR at 3T

Seventeen mongrel dogs (12 females, 5 males; mean initial weight = 26 ± 4 kg) with different durations of AF induced by rapid atrial pacing (RAP) (15,16) were included in this study. These canines with chronic AF were part of a separate longitudinal study aimed at characterizing the relationship between AF and cardiac pathophysiology. For more details on the pacemaker implantation and other procedures conducted to induce AF, see reference (15). In this study, disease duration is defined as the duration since the onset of RAP, since this is the starting point at which cardiac physiology is altered.

As summarized in Table 1, 17 dogs exhibiting different disease durations (0–22 months; mean RAP = 205.8 ± 187 days) were included in this study, for a total of 46 CMR sessions. Canines were fasted for a minimum of 12 hours before MRI. Animals were anesthetized with propofol (5–8 mg/kg, IV) for intubation and subsequently ventilated and maintained in a surgical plane of anesthesia with 1.5–3% isoflurane. Ventilation was controlled using a ventilator (DRE Premier XP MRI-Compatible Veterinary Anesthesia Machine, DRE Veterinary, Louisville, KY). Breath-hold MRI acquisition was performed with the ventilation suspended. Each animal was electrically cardioverted approximately 30 min prior to MRI setup. Note that cardiac T₁ mapping and cine MRI methods were performed at least 1 hour after cardioversion to minimize the effects of myocardial stunning. Heart rate, core body temperature, blood pressure, end-tidal CO₂, and oxygen saturation were continuously monitored and maintained within normal ranges. Blood was drawn during the MRI exam for hematocrit calculation. Pacemaker implantation and imaging (see below) were performed in accordance with protocols approved by the Institutional Animal Care and Use Committee at the University of Utah.

Table 1.

Summary of animal gender, initial weight, and MRI date with respect to days since RAP. 0 day corresponds to before inducing RAP. Mean RAP = 205.8 ± 187 days.

Canine Number	Gender	Initial weight (kg)	MRI date, RAP (days)
1	F	24.0	443, 478
2	M	32.7	268, 331, 401, 464, 539, 616
3	M	30.0	152, 180, 215, 262, 334, 418, 663
4	M	31.1	167, 223, 286, 349, 423
5	F	26.0	33
6	F	24.0	0, 97, 203, 412
7	F	22.7	0, 130, 164, 352
8	F	19.0	0, 37, 135, 164
9	F	23.0	0
10	F	26.9	0, 109, 230
11	F	27.2	0
12	F	25.2	0, 113
13	M	30.8	0
14	M	25.2	0
15	F	28.2	27, 48
16	F	27.1	0
17	F	29.0	0

CMR Hardware

CMR was performed on two 3T whole-body MRI scanners (Tim Trio and Verio, Siemens Healthcare, Erlangen, Germany) equipped with a gradient system capable of achieving a maximum gradient strength of 45 mT/m and a slew rate of 200 T/m/s. Among a total of 46 CMR scans, 32 and 14 scans were conducted on the Verio and Tim Trio scanners, respectively. The radio-frequency excitation was performed using the body coil. For the experiments conducted on the Tim Trio system, a 6-element body matrix coil array and a spine coil array (with 6 coil elements) were employed for signal reception. For the experiments conducted on the Verio system, a 32-element cardiac coil (RAPID MR International, Columbus, OH) was used for signal reception.

CMR Protocol

For cardiac T₁ measurements, we used the arrhythmia-insensitive rapid (AIR) cardiac T₁ mapping pulse sequence (17). Briefly, the AIR cardiac T₁ mapping pulse sequence acquires two single-shot balanced steady-state of free precession (b-SSFP) images: 1) a proton density-weighted image and 2) a T₁-weighted image. For more details on the AIR cardiac T₁ mapping pulse sequence, see reference (17). To minimize the impact of variation in contrast agent dosage and specific delayed imaging time, we measured the weight of each animal immediately prior to MRI, administered exactly 0.15 mmol/kg of gadobenate dimeglumine (Gd-BOPTA) (MultiHance, Bracco Diagnostics Inc., Princeton, NJ), and performed cardiac T₁ mapping pre-contrast and at exactly 15 min following Gd-BOPTA administration. We assumed that 15 min after bolus injection of Gd-BOPTA is equilibrium (4,18).

The AIR cardiac T₁ mapping pulse sequence was performed with the following imaging parameters: field of view = 260 mm × 195 mm, slice thickness = 8 mm, image acquisition matrix = 192 × 144, generalized autocalibrating partially parallel acquisitions (GRAPPA) (19) acceleration factor 1.8, flip angle = 35°, saturation-recovery time delay (TD) = 600 ms, receiver bandwidth = 930 Hz/pixel, and temporal resolution = 217 ms. In this study, we also used "paired" consecutive phase-encoding steps in centric k-space ordering to minimize b-SSFP image artifacts due to eddy currents (20). We acquired 3 short-axis slices (base, mid-ventricular, apex) in a single breath-hold duration of 6–9 heart beats (depending on heart rate). Using cardiac long-axis views as guides, for each CMR session, we defined the mid-ventricular short-axis plane as the mid-point between the mitral valve plane and apical cap, the basal short-axis plane as the mid-point between the mid-ventricular plane and mitral valve plane, and the apical short-axis plane as the mid-point between the mid-ventricular plane and apical cap.

As a secondary analysis, we also quantified LV functional parameters from retrospective ECG-gated breath-hold cine MR data acquired with b-SSFP readout (21,22). We imaged the whole heart with a stack of short-axis planes with the following imaging parameters: field of view = 260 mm × 195 mm, image acquisition matrix = 192 × 144, GRAPPA parallel imaging acceleration factor = 1.8, slice thickness = 7 mm, flip angle = 40–45°, receiver bandwidth = 1000 Hz/pixel, echo time (TE) = 1.3 ms, repetition time (TR) = 3 ms, 12–14 short-axis slices (with zero gap), spatial resolution = 1.4 mm × 1.4 mm, number of k-space lines per cardiac phase = 7, temporal resolution = 21 ms, and reconstructed cardiac frames = 25.

CMR Image Analysis

For cardiac T₁ quantification, we calculated the pixel-by-pixel T₁ map by dividing the T₁-weighted image, I_T1W, by the proton-density-weighted image, I_PD, to correct for the unknown equilibrium magnetization, M₀, and then solving the Bloch equation governing T₁ relaxation describing the ideal saturation-recovery experiment (17):

$$\frac{I_{T1w}}{I_{PD}}=\frac{M_0(1-e^{-TD/T_1})}{M_0}=(1-e^{-TD/T_1}),T_1=\frac{-TD}{\text{In}(1-\frac{I_{T1w}}{I_{PD}})}$$

Customized software in MATLAB was used to manually segment the myocardial contours and LV blood pool for each image (i.e., independent contour tracings for native and post-contrast T₁ maps per imaging plane per animal per CMR session). Care was taken to avoid partial volume averaging for each contour tracing. AIR cardiac T₁ data were randomized for analysis by the first reader (KH), who was blinded to the cine MR and histology results. Myocardial and blood T₁s were averaged within their respective contours for each of three LV locations (base, mid, apex). Myocardial ECV was calculated according to (13): (1−hematocrit) × (ΔR_1,m/ΔR_1,b) × 100%, where R_1,m is T₁⁻¹ of myocardium, and R_1,b is T₁⁻¹ of blood, and is Δ the difference between post-contrast and native.

For cardiac functional assessment, short-axis cine MR images were analyzed using Argus software (Siemens Healthcare, Erlangen, Germany). Another reader (MK), who was blinded to the T₁ and histology results, manually segmented the endocardial and epicardial LV contours in end-diastole and end-systole to calculate the following LV functional parameters: end diastolic volume (EDV), end systolic volume (ESV), stroke volume (SV), LV ejection fraction (LVEF), and LV mass. We note that papillary muscle was excluded for volume measurements, whereas it was included for mass measurement. The interventricular septum was included as part of the LV, while the left ventricular outflow tract was excluded.

Histological Analysis

Six canines enrolled in this study were terminated at their scheduled disease duration (0, 164, 435, 478, 686, and 690 days since the onset of RAP). Post-mortem tissue specimens (approximately 1 cm × 1 cm × 1 cm in size) were extracted from mid-ventricle of the antero-lateral LV wall of excised hearts fixed in 10% buffered formalin. Tissues were processed according to standard pathology protocol (i.e., tissues fixed in formalin, embedded in paraffin blocks, and sectioned at 4–6 µm). On average, a total of 6–10 sections were prepared with Masson’s Trichrome stain to quantify collagen volume fraction (CVF). In a consensus fashion, light-microscopic examination was performed by two experienced pathologists (ECH, RL), who were blinded to animal history, disease duration, and CMR results. The optimal histologic areas were identified visually by consensus, and the resulting digital images were captured using Infinity 2 microscope digital camera (Lumenera, Ottawa, Ontario, Canada) at 100× magnification. Each digital image was evaluated for CVF quantification using customized segmentation software. This software requires user input to learn the patterns, intensity, and color for inclusion (myocytes and interstitial fibrosis) and exclusion (red blood cells, blood vessels, and adipose tissue) elements. We used several training slides of LV tissues with Masson’s Trichrome stain to establish the inclusion and exclusion classification criteria. After training, the software generates automatically a classification mask by detecting fibrosis and myocyte pixels based on the trained inclusion and exclusion classification criteria. The two pathologists then compared the resulting classification mask to the original digital Trichrome image, in order to visually confirm the accuracy of the software in identifying the appropriate elements. Finally, using the resulting classification mask (see Figure 2), CVF was calculated as the pixel count of fibrosis (blue) divided by that of myocytes (red).

Figure 2.

Post-mortem LV samples with Masson's trichrome stain (top row) and the resulting classification masks (bottom row) used to calculate CVF: RAP = 0 (left) and RAP = 22 months (right). All specimens displayed with 100× magnification. Red: myocyte; blue: interstitial fibrosis; white: interstitium (and additional artificial space introduced during histologic slide preparation).

Statistical Analysis

For statistical analysis of temporal changes in CMR parameters, we examined the association between the repeated measurements of these parameters and disease duration using a linear-mixed-effect model (LMEM) (23) (N = 46). The exchangeable correlation structure was used to specify the correlation structure among the repeated measurements within each animal. This longitudinal study has an unbalanced distribution of repeated measurements for the different animals. We resolved this issue with LMEM, where the repeated measurements within each animal are treated as a cluster of observations. All LMEMs were fitted with the random intercept for each animal and disease duration as the fixed effect, where p value < 0.05 was considered significant. Individual trajectories of CMR parameters were plotted over disease duration. For the temporal change analysis, myocardial and blood T1 values were averaged over the basal and mid-ventricular short-axis planes. Apical short-axis plane results were excluded for the temporal change analysis, because they may be susceptible to partial volume averaging effects. We note that animals with only one data point (dogs 5, 9, 11, 13, 14, 16, 17) were excluded for the temporal change analysis (but included for other analyses).

For statistical analysis of post-mortem LV tissues, we examined the association between histologic quantification of CVF and disease duration using linear regression (N = 6), where p value < 0.05 was considered significant.

We also performed the following secondary analyses. To investigate whether there are regional differences in T₁ measurements between 3 different short-axis planes, we performed analysis of variance (ANOVA) for native myocardial T₁, native blood T₁, post-contrast myocardial T₁, and post-contrast blood T₁ measurements. To investigate whether the two different 3T MRI scanners produce different T₁ results, we performed a two-sample t-test (two-tailed) for native myocardial and blood T₁ measurements only, since the native T₁ measurements did not change significantly over disease duration and allows a comparison across different animals (see Table 2). To investigate whether electrical cardioversion induces myocardial edema, in 5 animals, we performed native T₁ mapping before and 1 hour after cardioversion and compared their native T₁ measurements. A paired t-test (two-tailed) was used to compare native myocardial T₁ between pre and post-cardioversion (N = 5), where p value < 0.05 was considered significant.

Table 2.

LMEM statistics to estimate the temporal changes of CMR parameters. We note that the T1 and ECV measurements are averaged over the basal and mid-ventricular short-axis planes. CI: confidence interval; SE: standard error. Mean RAP = 205.8 ± 187 days. For graphical displays of these parameters, see Figure 3. Percent change is defined as 100% × (final-initial)/initial.

CMR Variable	Estimated change per month (95% CI)	SE	p-value	Percent change
Native myocardial T1	0.44(−1.97, 2.86) ms	1.18	0.71	0.7 %
Native blood T1	−1.17(−6.04, 3.96) ms	2.38	0.62	−1.3 %
Post-contrast myocardial T1	−8.11 (−11.31, −4.92) ms	1.55	<0.001	−24.9 %
Post-contrast blood T1	−7.89 (−10.98, −4.79) ms	1.50	<0.001	−43.8 %
ECV	0.049 (−0.064, 0.16) %	0.06	0.38	4.5 %
EDV	0.21 (−0.37, 0.78) ml	0.28	0.47	−37.2 %
ESV	0.55 (−0.08, 1.18) ml	0.31	0.09	5.5 %
SV	−0.18 (−0.56, 0.20) ml	0.19	0.34	22.2 %
LVEF	−0.58 (−1.05, −0.10) %	0.23	0.02	−12.2 %
LV Mass	−0.19 (−0.61, 0.22) g	0.20	0.34	−3.9 %

Results

Figure 1 shows representative cardiac T₁ maps in three short-axis planes of a dog at 464 days since the onset of RAP, illustrating good data and contour tracing quality used in this study. Figure 2 shows post-mortem LV specimens of two different dogs with Masson's trichrome staining at baseline and 22 months after the onset of RAP, as well as their corresponding classification masks used to quantify CVF. These LV specimens and classification masks illustrate good data quality used in this study.

Figure 1.

Representative post-contrast cardiac T₁ maps (top row) and the same maps with contour tracings (bottom row), illustrating good data and contour tracing quality used in this study: basal (left), mid-ventricular (middle), and apical (right) planes.

In 17 animals observed over disease duration ranging from 0–22 months (mean RAP = 205.8 ± 187 days), post-contrast myocardial T₁ decreased significantly from 872 to 698 ms (Fig. 3; p < 0.001), which corresponds to a 24.9% relative reduction. In contrast, ECV increased from 21.0 to 22.0% (p = 0.38), which corresponds to only a 4.5% relative increase. This discrepancy was partially investigated by histologic analysis of post-mortem LV tissues. In 6 different dogs sacrificed at different disease duration ranging from 0–22 months, CVF quantified by histology increased from 0.9 to 1.9% (Fig. 3; slope = 0.044, bias = 1.2%, correlation coefficient = 0.30; p = 0.56), which agreed more with ECV than post-contrast myocardial T₁.

Figure 3.

Plot of the estimated regression line (solid line), along with the 95% confidence intervals (dashed lines), describing the temporal changes in the following parameters: native myocardial T1 (a), native blood T1 (b), post-contrast myocardial T₁ (c), post-contrast blood T₁ (d), ECV (e), and histologic quantification of CVF (f), LVEF (g), ESV (h), EDV (i), SV (j), LV mass (k). For the statistics, see Table 3.

We examined other parameters to identify the cause for this discrepancy. Among cardiac T₁ data (native myocardial T₁, native blood T₁, post-contrast blood T₁), only post-contrast blood T₁ decreased significantly from 578 to 402 ms (Fig. 3; p < 0.001), which corresponds to a 43.8% relative reduction. Note that the temporal trends in post-contrast myocardial and blood T₁s are similar. Both native myocardial T₁ and native blood T₁ did not change significantly (Table 2). Among LV functional parameters (EDV, ESV, SV, LVEF, and LV mass), only LVEF decreased significantly from 46.1 to 33.6% (p = 0.02; Table 2), which corresponds to a 37.2% relative reduction, suggesting progressively worsening LV dysfunction.

The results from the secondary analyses (regional variations in T₁, inter-scanner variability of native T₁, comparison of native myocardial T₁ between pre and post-cardioversion) were as follows. In all animals, the mean native cardiac T₁, native blood T₁, post-contrast cardiac T₁, and post-contrast blood T₁ values were not different between basal, mid-ventricular, and apical short-axis planes (see Table 3 for the statistics). According to a two-sample t-test (two-tailed), only mid-ventricular and basal myocardial T₁ measurements were significantly different between two different 3T MRI scanners (p < 0.05; see Table 4), but note that the magnitude of the percent change was less than 4% for each parameter (i.e., negligible). In 5 animals, the mean native myocardial T₁ before (1374 ± 34 ms) and 1 hour after cardioversion (1399 ± 27 ms) was not different (p = 0.36), which corresponds to only a 1.8% relative increase.

Table 3.

Comparison of mean native myocardial T₁, native blood T₁, post-contrast myocardial T₁, and post-contrast blood T₁ values for the basal, mid-ventricular, and apical short-axis planes. According to ANOVA, all four T₁ measurements were not different among the three short-axis planes. Mean RAP = 205.8 ± 187 days.

CMR variable	Base	Mid	Apex	p-value
Native myocardial T1	1383 ± 44 ms	1372 ± 47 ms	1390 ± 53 ms	0.19
Native Blood T1	1725 ± 105 ms	1719 ± 104 ms	1692 ±89 ms	0.29
Post-contrast myocardial T1	831 ± 74 ms	811 ± 82 ms	823 ± 81 ms	0.71
Post-contrast Blood T1	527 ± 77 ms	521 ± 82 ms	534 ± 80 ms	0.54

Table 4.

Comparison of mean native myocardial and blood T₁ values across two different 3T MRI scanners. According to a two-sample t-test (two-tailed), only mid-ventricular and basal myocardial T₁ measurements were significantly different (p < 0.05), but note that the magnitude of the percent change was less than 4% for each parameter (i.e., negligible). Percent change is defined as 100% × (Trio-Verio)/Verio.

CMR variable	Verio	Tim Trio	p-value	Percent change
Native apical myocardial T1	1396 ± 57 ms	1379 ± 42 ms	0.35	−1.2 %
Native mid-ventricular myocardial T1	1386 ± 43 ms	1340 ± 40 ms	0.002	−3.3 %
Native basal myocardial T1	1399 ± 36 ms	1348 ± 42 ms	<0.001	−3.7 %
Native apical blood T1	1689 ±82 ms	1700 ± 106 ms	0.73	0.6 %
Native mid-ventricular blood T1	1728 ± 108 ms	1700 ± 97 ms	0.41	−1.6 %
Native basal blood T1	1730 ± 109 ms	1713 ± 100 ms	0.63	−1.0 %

Discussion

In this study, in canine models with chronic AF (0 – 22 months), we measured the temporal changes in post-contrast myocardial T₁ and myocardial ECV, both of which have been associated with interstitial fibrosis (4,7–12). In the observed canines, post-contrast myocardial T₁ decreased significantly over disease duration, whereas myocardial ECV did not. Histologic quantification of CVF in a subset of animals suggests a non-significant difference in CVF over disease duration, which agrees more with ECV than post-contrast myocardial T₁. We note that a more comprehensive study, including histologic, cardiac, and renal functional analyses, is warranted to test rigorously which CMR parameter (ECV or post-contrast myocardial T₁) agrees more with CVF. Interestingly, post-contrast blood T₁ decreased significantly over disease duration, suggesting that decreasing post-contrast myocardial T₁ may have been largely driven by decreasing post-contrast blood T₁. This observational study is the first report of conflicting findings between post-contrast myocardial T₁ and ECV in the context of a longitudinal study. The results of this study agrees with a prior cross-sectional study (9), which first reported a weak association between post-contrast myocardial T₁ and ECV in a sub-analysis.

This study also highlights the benefits of myocardial ECV over post-contrast myocardial T₁. In the 17 animals studied over disease duration ranging from 0–22 months with an identical AIR cardiac T₁ mapping protocol (MRI at exactly 15 min after administration of 0.15 mmol/kg of MultiHance), we observed decreasing trends in post-contrast myocardial T₁ and post-contrast blood T₁. These temporal trends suggest that decreasing post-contrast myocardial T₁ may have been largely driven by decreasing post-contrast blood T₁. Our cine MRI data showed that LVEF decreased significantly with disease duration, and it is plausible that a reduced cardiac output could lead to higher concentration of contrast agent in the blood at 15 min after administration (i.e., since less blood is delivered to the kidney per unit time). We note that the separate longitudinal study design did not anticipate renal function to be influenced by chronic AF. Unfortunately, due to the nature of this study (retrospective analysis of data collected from a separate longitudinal study), we are unable to measure changes in renal function over disease duration. Consequently, we are unable to draw a conclusion on what caused the decreasing trend in post-contrast blood T₁ over disease duration. Despite the lack of renal functional data and limited histologic data, our study suggests that CVF agrees better with ECV than post-contrast myocardial T₁.

It should be noted that canine T₁ measurements reported in this pre-clinical study may not be translatable to other pre-clinical studies using different CMR protocols. A variety of factors, such as field strength, contrast agent type and dosage, specific delayed imaging time, pulse sequence type, heart rate and rhythm, may influence the accuracy of T₁ measurements. Therefore, CMR researchers must be careful when translating T₁ values presented in this preclinical study into their own studies.

Study Limitations

A limitation of this study is that the enrolled canines were part of a separate longitudinal study. Consequently, our animal data had an unbalanced distribution of repeated measurements (see Table 1). We used LMEM to account for the unbalanced distribution of repeated measurements. Another limitation of this study is that the histologic analysis was performed for only 6 animals (N=6), which were sacrificed according to the study objectives of the separate longitudinal study. In addition, for each animal, we analyzed a post-mortem tissue specimen of approximately 1 cm × 1 cm × 1 cm in size from the mid-ventricle of the antero-lateral LV wall to represent the whole LV. While it would be more comprehensive to analyze tissues from multiple locations of the LV, this study did not have access to the rest of the heart. A more comprehensive histological evaluation (e.g., biopsy over disease duration) is warranted to confirm our CMR findings. However, we note that performing longitudinal biopsies would add considerable expense and procedural risk to the separate longitudinal study. Furthermore, biopsy samples of the right ventricular septum are small in size (~1–2 mm in length), thereby harder to process and sensitive to sampling errors. Another potential confounder is heart-rate variation in animals under anesthesia during MRI. We typically maintain a steady level of isoflurane during MRI, but the animals' heart rate varies within and across MRI sessions. Consequently, cardiac functional parameters may have been affected by variations in heart rate. Although we performed cine CMR at least one hour after cardioversion, cardiac functional parameters reported in this study may have been influenced by residual myocardial stunning.

Conclusions

This study shows that post-contrast myocardial T₁ and ECV may disagree in a longitudinal canine study. A more comprehensive study, including histologic, cardiac, and renal functional analyses, is warranted to test rigorously which CMR parameter (ECV or post-contrast myocardial T₁) agrees more with CVF.

List of Abbreviations

LV

left ventricle

LVEF

left ventricular ejection fraction

ECV

extracellular volume

CMR

cardiovascular magnetic resonance

RAP

rapid atrial pacing

ECG

electrocardiogram

b-SSFP

balanced steady state of free precession

AF

atrial fibrillation

R

acceleration rate

bpm

beats per minute

GRAPPA

generalized autocalibrating partially parallel acquisitions

T₁

longitudinal relaxation time

LGE

late gadolinium enhanced

TE

echo time

TR

repetition time

AIR

arrhythmia-insensitive-rapid

Gd-BOPTA

gadobenate dimeglumine

TD

saturation-recovery time delay

LMEM

linear-mixed-effect model

ANOVA

analysis of variance

EDV

end diastolic volume

ESV

end systolic volume

SV

stroke volume

CVF

collagen volume fraction